ZINC DIVISION: Another Century of Leadership: CEZinc Refinery
The world’s best zinc comes, naturally, from the world’s best zinc refinery. That honour arguably goes to CEZinc.
The refinery was built in 1963 at Salaberry-de-Valleyfield, Que. This small town southwest of Montreal was chosen for what plant metallurgist Gerry Leroux calls the “Three Ps: power, people & placement.” Ample electric power is available at a competitive price. After the closure of the local textile mills, many skilled workers remained in the area. Rail, river and road transport are available to deliver concentrates to the plant and for shipping zinc to customers.
CEZinc is the third largest zinc refinery in the Western World and turns out 5% of the annual global zinc production. Thanks to numerous expansions and last year’s optimization, plans call for the production of 280,000 tonnes of zinc in all forms this year. In addition, some 500 tonnes of cadmium, 4,000 tonnes of copper concentrate, and 460,000 tonnes of sulphuric acid will be made. Jarosite residue, rendered inert by the Jarofix process, will total 175,000 tonnes. Further expansions would depend on finding additional markets for sulphuric acid, so no major changes to plant size are planned.
In 1992, CEZinc earned an ISO 9002 certification, the first zinc refinery to do so. The standard covers all of the production facility with the exception of the cadmium circuit and sulphuric acid plants. Work is now underway on codifying environmental management practices according to ISO 14001 standards.
The Business of Zinc …
Basic zinc refining by roasting, leaching and electrolysis has been practised for many years. But CEZinc has moved beyond “basic” into cleaner, more efficient technology suitable for the 21st Century. This includes everything from attention to customer needs to a proactive environmental commitment.
Each year 520,000 tonnes of zinc concentrate arrives by rail at the plant and is unloaded with a backhoe. About 40% comes from the Brunswick mine and the remainder from seven or eight other Canadian mines. A recycling program adds rejects from other zinc plants, baghouse dust from the brass industry, and zinc oxide from the pulp and paper industry to the total.
Concentrates consist of zinc sulphide (32% S and 53% Zn). The feed is roasted in two 32-m2 and one 72-m2 and one 76-m2 Lurgi fluid bed roasters at 950C to produce zinc oxide (ZnO) and sulphur dioxide (SO2). The 9-11% SO2 gas is used to make sulphuric acid (H2SO4). There are three Monsanto acid plants, two single-adsorption and one double-absorption. Sulphur dioxide emissions have dropped from 35 kg/tonne of H2SO4 produced in 1966 to 10 kg/tonne last year. Current government regulation calls for a maximum of 15 kg/tonne. Calcine (ZnO) from the roasters is cooled, ground in a ball mill and stored ahead of leaching.
Roasted calcine undergoes a three-stage countercurrent leach process. The calcine all goes into the first leach tank of the neutral leach. Reagents are added as needed at different points in the process. Neutral leach conditions at pH 4.0 are maintained in the six tanks where, over a period of about two hours, ZnO is leached and iron is precipitated as ferric hydroxide (FeOH3). After thickening, the solution is sent on to the purification circuit, while the solids are releached in a low acid leach stage at pH 2.0. After another liquid-solid separation, the solution is recycled back to the neutral leach while the solids pass on to the jarosite circuit to complete the leaching process and simultaneously precipitate the iron as jarosite.
Zinc recovery is 96.5%, up from the historical level of 95%, due to increased neutral leach time and process control optimization.
In the first stage of purification, the impure solution from the neutral leach thickener is mixed at 70C with zinc dust to remove copper and cadmium from solution. The solution is filtered by Perrin plate-and-frame filters, steam heated, and sent to the second purification stage. Here the solution is mixed at 90C with steam, more zinc dust, copper sulphate (CuSO4) and antimony trioxide (Sb2O3). The purified solution is filtered again and stored prior to electrolysis. During the first process, deleterious elements such as cobalt and nickel are removed from the electrolyte. The zinc content increases by about 20 g/L to 160 g/L Zn through evaporation and zinc dust addition.
The residue from the purification circuit is a Zn-Cu-Cd-Co-Ni cake. It is repulped with filtrate from the purification filters and electrolyte. The copper-cadmium circuit produces a copper cement, which goes to the Horne or Kidd smelter, and cadmium metal, for which there is a local market.
The purified electrolyte is cooled and the precipitated gypsum removed from it. Electrolyte is mixed with strontium carbonate (SrCO3), gelatin and a frothing agent, then pumped to the electrowinning cells. There are three cellhouses at CEZinc. No.1 and No.2 are no longer used, and it is No.3, which was started up in 1990, that produces 280,000 tonnes of cathodic zinc annually. Each cell contains lead anodes and aluminum cathodes. Metallic zinc electroplates on the aluminum in sheets weighing 80 kg each. The spent electrolyte stream is either pumped to leaching and purification or cooled and returned along with purified electrolyte to the cells.
Cathode stripping, i.e, peeling off the zinc strip from the aluminum, is fully automated. Half the cathodes in each cell are removed and stripped per load, in this way preventing an open circuit in the cell. The entire cellhouse can be stripped in 48 hours. The zinc strip is 99.99% pure, with only 20 ppm Pb remaining.
Stripped cathodic zinc is fed by gravity directly from the stripping line to two electric induction furnaces. A small amount of ammonium chloride (NH4Cl) is added as a fluxing agent. Dross from the furnaces is returned to the roasters.
CEZinc has three continuous casting lines, one for 25-kg slabs and two for 1,100-kg jumbos. The refinery makes SHG (special high-grade) zinc or alloys with lead or aluminum for PW (prime western) or CGG (continuous galvanizing grade) products. Skimming the 500C surface of the zinc in the moulds remains the only manual operation in the casting line and will be automated later this year. The jumbo moulds have heated covers so that the shapes solidify from the bottom to the top, leaving no voids inside. Zinc shot is also produced by forcing a stream of molten zinc through an underwater grid. Zinc dust is recycled to the electrolyte purification circuit. Annual production is divided thus: 80,000 tonnes of slabs and powder; 135,000 tonnes of jumbos; and 45,000 tonnes of shot.
… Not Polluting
“We’re not in the business of polluting, we’re in the business of selling zinc,” says Leroux. That’s why the Jarofix process is such a “quantum leap” in residue management. It is a process in which Noranda has a lot of faith; half of the $50-million in capital expenditures in 1999 at CEZinc went towards implementing this unique technology.
All zinc refineries produce a residue slurry containing iron; in CEZinc’s case this is jarosite. This slurry has the potential to leach into the environment over time, and as such is stored in lined, monitored ponds. Only recently has it become economic to convert jarosite slurry into an inert solid, and this solid can be safety stored above ground.
Prior to implementing the Jarofix process, the leach plant was modified to produce sodium-jarosite. Leach circuit residue is filtered on two 20-m-long filters and washed. The pH of the residue is adjusted with lime, and cement is added to produce a stable, inert solid. Curing starts as the residue is trucked to the storage area, one kilometre east of the refinery. The site is landscaped with low stone walls, trees and grass. The residue pile will be vegetated so as to blend in with the countryside. Although there may be many potential uses for the inert residue in the construction industry, Noranda will contain all of it at the CEZinc site. In that way the company can monitor it and guarantee its stability.
The CEZinc site includes enough permitted land to operate for the next 75 years, more than any other zinc refinery in the world. That fact points toward the long-term view held by the company. In 1998 a jardin industriel was officially opened on its land adjacent to the old jarosite ponds. There are walkways, bicycle paths, and pleasant landscaping for the enjoyment of all. The city has assumed responsibility for upkeep, but Noranda continues to own the property.
CEZinc is the recipient of several awards. It has received a federal-provincial Certificate of Acknowledgement for its efforts to clean up the water in the nearby St. Lawrence River. It implemented a $4.5-million selenium capture process, which prevents that metal from reaching the waterway. There is also a new $17-million effluent treatment plant for the removal of heavy metals. Such improvements, as well as leach circuit optimization, the jumbo casting line, and improved plant operating efficiency, won CEZinc the Noranda President’s Technical Award in 1999. The company holds several awards from CN Rail and Conrail for the safe shipment of acid.
These accomplishments plus its forward-looking attitude ensure CEZinc a leading role in zinc production for a long time to come.